Semiconductor light-emitting devices (LEDs) are among the most efficient light sources currently available. Materials systems currently of interest in the manufacture of high-brightness LEDs capable of operation across the visible spectrum include Group III-V semiconductors, particularly binary, ternary, and quaternary alloys of gallium, aluminum, indium, and nitrogen, also referred to as III-nitride materials. Typically, III-nitride light emitting devices are fabricated by epitaxially growing a stack of semiconductor layers of different compositions and dopant concentrations on a sapphire, silicon carbide, or III-nitride substrate by metal-organic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), or other epitaxial techniques. The stack often includes one or more n-type layers doped with, for example, Si, formed over the substrate, a light emitting or active region formed over the n-type layer or layers, and one or more p-type layers doped with, for example, Mg, formed over the active region.
The active region is often a single quantum well layer, or multiple quantum well layers separated by and sandwiched between layers of semiconductor materials with larger bandgap energies than the quantum well layers. The larger bandgap energy layers that separate the quantum well layers are often referred to as barrier layers. The larger bandgap energy layers between which the active region is located are often referred to as cladding or confinement layers. Other layers may be located between the confinement layers and the active region. The barrier and confinement layers provide barriers to the diffusion of charge carriers away from the active region.
FIG. 1 illustrates an example of an active region sandwiched between two confinement layers, as described in U.S. Pat. No. 6,046,464. A GaN active region 112 is located between two confinement layers 114a and 114b of Al1-xGaxN. Aluminum is often used in confinement layers because the presence of aluminum in a III-nitride semiconductor layer typically increases the bandgap of that layer, providing for good carrier confinement.
The use of AlGaN confinement layers creates several problems.
First, it is difficult to achieve the required hole concentration in Mg-doped AlGaN. One reason is that the activation energy for Mg increases as the composition of Al increases in AlGaN. It has been observed that in GaN, only about 1% of the Mg incorporated is activated at room temperature. It has also been observed that the activation energy of a dopant increases as the bandgap energy of the host material increases. Therefore, the percentage of activated Mg atoms in AlGaN is expected to be less than 1%. This means that higher concentrations of Mg atoms must be incorporated into the AlGaN layer in order to achieve the required hole concentrations. Requiring high Mg concentration in AlGaN has two disadvantages. First, it is difficult to incorporate high Mg concentrations in AlGaN during growth. Second, the presence of high dopant concentrations can undesirably affect the quality and electronic properties of the AlGaN single crystal film.
Second, large polarization fields exist at the GaN/AlGaN interface. These polarization fields are caused by the different electronegativities of Al, In, Ga, and N, as well as the asymmetric nature of the wurtzite crystal structure present in III-nitride LEDs. The polarization fields essentially produce a sheet charge between the GaN and AlGaN interface, which moves the energy band diagram up or down, depending on the polarity of the sheet charge at the interface. At the GaN/AlGaN interface, a positive sheet charge exists and pulls the conduction energy band down, which reduces the effectiveness of AlGaN as a confinement layer. The effect of polarization fields is illustrated in FIG. 2, which is an energy band diagram of a device with an AlGaN confinement layer. As illustrated in FIG. 2, the bandgap of quantum well layer 310 is less than the bandgap of cap layer 302, which is less than the bandgap of confinement layer 304. However, the polarization field at the cap layer/confinement layer interface has pulled the energy diagram at point 308 down below the quasi-Fermi level 306. The quasi-Fermi level is most easily understood as the energy level below which charge carriers can reside at T=0. Because the conduction band is pulled down at point 308, the voltage needed for similar current densities is increased. This decreases the energy barrier and the electron confinement of the AlGaN layer.
Third, growth of AlGaN requires higher temperatures than other GaN-based layers. One way to change to a higher growth temperature is to initiate a pause in the growth. Growth pauses are generally undesirable because they allow impurities to accumulate on the surface of the crystal, which can degrade the quality of the crystal. In addition, the high temperature required for AlGaN can undesirably impact the material properties of the layers in the active region.
In accordance with the invention, a III-nitride light emitting device includes a substrate, a first conductivity type layer overlying the substrate, a spacer layer overlying the first conductivity type layer, an active region overlying the spacer layer, a cap layer overlying the active region, and a second conductivity type layer overlying the cap layer. The active region includes a quantum well layer and a barrier layer containing indium. One of the spacer layer and the cap layer contain indium. The barrier layer may be doped with a dopant of first conductivity type and may have an indium composition between 1% and 15%. In some embodiments, the light emitting device includes a lower confinement layer formed between the first conductivity type layer and the active region. The lower confinement layer may be doped with a dopant of first conductivity type and may have an indium composition between 0% and 15%. In some embodiments, the light emitting device includes an upper confinement layer formed between the second conductivity type layer and the active region. The upper confinement layer may be doped with a dopant of second conductivity type and may have an indium composition between 0% and 15%. The cap layer may be doped with a dopant of second conductivity type and may have an indium composition between 0% and 15%. The spacer layer may be doped with a dopant of first conductivity type and may have an indium composition between 0% and 15%.